In the manufacture of composite panels such as used as interior trim panels in cars, vans and trucks, it is desirable that these panels be lightweight and have a skin or cover with a soft backing supported on a relatively hard substrate or core for mounting in the vehicle interior. Many various methods have been used in the past to form such panels and they generally require several steps which play a substantial factor in their costs, However, there is a method for making such a panel in one simple step.
In the above referred to single step method, an assembly of foam layers is built up comprising a core-forming foam layer, an intermediate foam layer, and a cover-backing soft foam layer with a scrim sheet under the core-forming foam layer and a cover sheet over the cover-backing foam layer. The core-forming layer is impregnated with a hardening compound coating the cells walls of the foam that is activated with heat. The intermediate foam layer and the cover-backing foam layer may also be impregnated with the hardening compound but in lesser amounts. The panel is formed by subjecting this layered assembly to heat and pressure in a single compression molding operation at a temperature that activates the hardening compound. The layers of the assembly are bonded together either by the hardening compound or by thermoplastic films that are inserted between the layers and are melted by the heat of the molding operation and then become adhesive. In the molded panel, the foam core layer becomes relatively hard and self-supporting, the foam cover-backing layer remains relatively soft and compressible to the touch, and the intermediate foam layer becomes not as hard as the foam core layer and not as soft as the cover-backing layer. The relative softness or hardness of the layers can be varied as desired by the amount of the hardening compound with which they are impregnated. And the degree of softness or hardness can also be varied as desired by the thickness, density and compression of the foam layers. Furthermore, the panel may be formed consisting essentially of only a self-supporting foam core or substrate, a second foam layer and a cover sheet. This method and the panels made thereby are disclosed in U.S. Pat. No. 5,089,328 issued to Doerer et al.
While the above single step compression molding method and the panels made therefrom have proven generally satisfactory, it has been discovered that certain difficulties can occur when the cover layer or skin is a plastic sheet of vinyl or expanded vinyl as currently used for car and truck interior trim panels. In the compression molding of the layers, the vinyl skin is actually cold formed as no heat is introduced into the compression molding process except through the press mold to react the hardening compound in the foam core layer. When plastic is cold formed, it retains a significant amount of memory about its previous state. And upon removing the laminated layers from the mold, the now contoured plastic sheet attempts to revert to its original form (i.e. that of a plane sheet). This results in residual stresses in the panel to various degrees depending on the amount of the cold working required to obtain the desired panel contour and can cause the panel to warp when removed from the compression molding press dies. The core or substrate is not substantially stiff enough to prevent this warping from occurring where the cold drawing is of such a large because of the low density of its foam structure and, consequently, lower physical properties as compared with other core materials such as the thermosetting plastics used for trim panels.
Another problem with the use of a plastic sheet as the cover layer or skin is the occurrence of wrinkles. Wrinkles are typically eliminated by greater stretching or drawing of the plastic sheet during the compression molding through greater retention of the sheet by the die clamping means. And this greater stretching of the plastic sheet increases the residual stresses created during the cold forming resulting in an even greater tendency to cause panel warpage. Moreover, it is not normally possible to stretch the plastic skin without also stretching the other layers of the laminate assembly. And because these other layers are not as elastic as the plastic sheet, they are prone to tear much easier when attempting to remove the wrinkles in the plastic skin by greater stretching.